Silicon carbide composite with coated fiber reinforcement

ABSTRACT

A composite is comprised of reinforcement fibers having a continuous coating with a first layer of a metal oxide wherein the metal is from the group consisting of aluminum, yttrium, titanium, zirconium, beryllium, silicon, and the rare earths, and a molten silicon infiltration formed silicon carbide matrix. The coating may have a second layer from the group consisting of rhodium, iridium, metal carbide, metal silicide, metal nitride, and metal diboride, on the metal oxide coating. The reinforcement fibers being fibers from the group consisting of elemental carbon, silicon carbide, and mixtures thereof. A process for producing the fiber reinforced composite comprises depositing on the fibers a continuous coating comprised of the first layer of the metal oxide, and the second layer. A carbonaceous material is admixed with the coated fibers so that at least 5 volume percent of the mixture is the fibers. The mixture is formed into a preform having an open porosity ranging from about 25 volume percent to about 90 volume percent of the preform. The preform is heated in an inert atmosphere or partial vacuum, and infiltrated with molten silicon to produce an infiltrated product having the composition of the composite.

This application is a File Wrapper Continuation of Ser. No. 08/215,246,filed Mar. 21, 1994, now abandoned, which is a Division of Ser. No.08/059,843, filed May 11, 1993, now abandoned, which is a Continuationof Ser. No. 07/716,443, filed Jun. 17, 1991, now abandoned.

This application is related to copending applications Ser. No.07/056,516, filed Jun. 1, 1987 now U.S. Pat. No. 5,015,540, Ser. No.07/001,806, filed Sep. 24, 1987 now U.S. Pat. No. 4,766,297, Ser. No.07/714,417, filed Jun. 12, 1991, (Atty. docket RD-20,713) now abandoned,and Ser. No. 07/716,442, filed Jun. 17, 1991, (Atty. docket RD-20,942)now abandoned, and Ser. No. 07/716,443, filed Jun. 17, 1991, (Atty.docket RD-20,769) now abandoned.

This invention relates to a composite and method for forming thecomposite, comprised of coated silicon carbide fibers in a matrixcontaining phases of silicon carbide.

U.S. Pat. Nos. 4,120,731; 4,141,948; 4,148,894; 4,220,455; 4,238,433;4,240,835; 4,242,106; 4,247,304; 4,353,953; 4,626,516; 4,889,686; and4,944,904; assigned to the assignee hereof and incorporated herein byreference, disclose silicon infiltration of materials which includecarbon, molybdenum, carbon-coated diamond and/or cubic boron nitride,and blends of carbon with silicon carbide, boron nitride, siliconnitride, aluminum oxide, magnesium oxide and zirconium oxide.

High temperature fiber reinforced composites have great potential foruse in aircraft and gas turbines due to the high strength to weightratio of such materials. Composites of carbon fiber reinforced carbonmatrices have been used in aircraft construction, but poor oxidationresistance has limited use to low temperature applications of 1000° C.or less. High strength carbon fibers have been infiltrated with moltensilicon to provide a silicon matrix for protecting the carbon fiberreinforcements. However, the silicon infiltration converts the carbonfiber reinforcements into relatively weak, irregular columns of siliconcarbide crystals resulting in composites with low toughness andrelatively modest strength.

As an alternative approach, attempts have been made to incorporatesilicon carbide fibers in a silicon matrix by the process of siliconinfiltration. Unfortunately, silicon carbide has a limited solubility inmolten silicon, and leads to transport and recrystallization of siliconcarbide causing the silicon carbide fibers to loose substantialstrength. Also, silicon carbide forms a strong bond with silicon so thefiber bonds to the matrix resulting in brittle fracture of thecomposite. In ceramic composites, a relatively weak bond at thefiber-matrix interface is preferred in order to achieve improvedfracture toughness. Toughness is improved in fiber reinforced compositeswhen the fiber reinforcement does not bond with the surrounding matrix,so that force applied to the matrix is transferred from the matrix tothe fiber substantially by friction.

It is an object of this invention to provide infiltration formed fiberreinforced composites having improved toughness and oxidationresistance.

It is another object of this invention to provide infiltration formedfiber reinforced composites having protective coatings for the fibers.

It is another object of this invention to provide a method of forminginfiltration formed fiber reinforced composites having improvedtoughness and oxidation resistance.

BRIEF DESCRIPTION OF THE INVENTION

Composites having improved toughness are comprised of reinforcementfibers having a continuous coating with a first layer of a metal oxidewherein the metal is from the group consisting of aluminum, yttrium,titanium, zirconium, hafnium, beryllium, silicon, lanthanum, scandiumand the rare earths, and mixtures thereof, and a molten siliconinfiltration formed silicon carbide matrix. The fiber coating may have asecond layer from the group consisting of metal carbide, metal silicide,metal nitride, metal diboride, and a high melting noble metal such asrhodium, and iridium, on the metal oxide layer. The reinforcement fibersare from the group consisting of elemental carbon, silicon carbide, andmixtures thereof. The metal oxide coating protects the reinforcementfibers during molten silicon infiltration forming of the matrix so thatthe fiber has a desirable debonding from the matrix. The metal oxidecoating also reduces reaction between the matrix and fibers during hightemperature service. The second layer protects the reinforcement fibersduring infiltration forming of the matrix so that the fiber has adesirable debonding from the matrix.

A process for producing tough fiber reinforced composites having fibersfrom the group consisting of elemental carbon, silicon carbide, andmixtures thereof, comprises depositing a continuous coating with a firstlayer of a metal oxide wherein the metal is from the group consisting ofaluminum, yttrium, titanium, zirconium, hafnium, beryllium, silicon,lanthanum, scandium and the rare earths, and mixtures thereof, and asecond layer from the group consisting of a metal that reacts withsilicon to form a silicide, metal carbide, metal silicide, metalnitride, metal diboride, and a high melting noble metal such as rhodium,and iridium, on the metal oxide layer. The coated fibers are admixedwith a carbonaceous material to form a mixture having at least 5 volumepercent of the fibers. The mixture is formed into a preform having anopen porosity ranging from about 25 percent by volume to about 90percent by volume of the perform. The preform is heated in an inertatmosphere or partial vacuum, and infiltrated with molten silicon toproduce an infiltrated product having the composition of the composite.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the terms "fiber or fibers" include fibers, filaments,whiskers, tows, cloth, and combinations thereof. The fibers to be coatedwith metal oxide are selected from the group consisting of elementalcarbon, silicon carbide and combinations thereof.

Reference herein to a fiber of silicon carbide includes presentlyavailable single crystal or polycrystalline fibers, or wherein siliconcarbide envelops a core, and which generally are produced by chemicalvapor deposition of silicon carbide on a core such as, for example,elemental carbon or tungsten. There are processes known in the art whichuse organic precursers to produce silicon carbide containing fiberswhich may introduce a wide variety of elements into the fibers. Examplesof known silicon carbide fibers are Nicalon silicon carbide fibers,Nippon Carbon, Japan, HPZ and MPDZ silicon carbide fibers, Dow Corning,and fibers having the trade name SCS-6, or SCS-0 produced by Textron,Mass. Additional information about silicon carbide fibers can be foundin "Boron and Silicon Carbide Fibers," T. Schoenberg, ENGINEEREDMATERIALS HANDBOOK Volume 1 COMPOSITES, ASM International, 1987, pp58-59, incorporated herein by reference.

Reference herein to fibers of carbon include amorphous, singlecrystalline or polycrystalline carbon fibers such as derived from thepyrolysis of rayon, polyacrylonitrile or pitch. The fibers to be coatedwith metal oxide are stable at the infiltration temperature used in theprocess. Preferably, the fibers have at room temperature, i.e. about 22°C., in air a minimum tensile strength of about 100,000 psi and a minimumtensile modulus of about 25 million psi. Additional information aboutcarbon fibers can be found in, "CARBON FIBERS," J. B. Donnet, O. P.Dahl, ENCYCLOPEDIA OF PHYSICAL SCIENCE AND TECHNOLOGY, Vol. 2, 1987, pp.515-527, incorporated herein by reference.

The fibers can be used as a continuous filament, or as discontinuousfibers, which frequently have an aspect ratio of at least 10, and in oneembodiment it is higher than 50, and yet in another embodiment it ishigher than 1000. The fibers are admixed with a carbonaceous material.Low aspect ratio fibers are preferred in a random mixture of the fibersand carbonaceous material, since the low aspect ratio fibers pack betterand produce high density preforms. On the other hand, in an orderedarray of fibers, high aspect ratio fibers are preferred since theyproduce composites with the highest density of reinforcement and thebest mechanical properties. Generally, the fibers range from about 0.3micron to about 150 microns in diameter, and from about 10 microns toabout 10 centimeters in length or longer. Frequently, the fiber iscontinuous and as long as desired.

Continuous fibers can be filament-wound to form a cylindrical tube, orformed into sheets by placing long lengths of fiber next to and parallelto one another. Such sheets can consist of single or multiple layers offilaments. Continuous filaments can also be woven, braided, or otherwisearrayed into desired configurations. When fibers are continuous or verylong the use of the term "aspect ratio" is no longer useful.

The fiber coating, i.e., the first and second layers, can be depositedby methods well known in the art that deposit a continuous coatingwithout damaging the fiber. Coating processes such as chemical vapordeposition, or physical vapor deposition processes such as sputteringare suitable. In addition, non-carbonaceous second layers can be coatedwith a third layer of carbon to promote wetting with the molten siliconinfiltrant. Elemental carbon can be deposited on the fibers by knownmethods, for example, in the form of pyrolytic carbon. A continuouscoating is deposited covering the entire surface of the reinforcementphase fibers, the ends of the fiber may be exposed but such exposure isnot considered significant. Preferably, the fiber coating is uniform andsmooth to minimize mechanical interlocking between the coating andmatrix. Additional information about such coating processes can befound, for example in, "Metallic & Ceramic Coatings: Production, HighTemperature Properties & Applications," M. G. Hocking, V. Vasantasree,P. S. Sidky, Longman Scientific & Technical, Essex England, 1989,incorporated herein by reference.

The coating second layer is from the group consisting of a metal thatreacts with silicon to form a silicide, metal carbide, metal silicide,metal nitride, metal diboride, and a high melting noble metal such asrhodium, and iridium. The metal carbide is a carbide of silicon,tantalum, titanium or tungsten; the metal silicide is a silicide ofchromium, molybdenum, tantalum, titanium, tungsten or zirconium; themetal nitride is a nitride of silicon, aluminum, titanium, zirconium,hafnium, niobium, tantalum, or boron; the metal diboride is a diborideof titanium, zirconium, hafnium, or aluminum; and the metal that reactswith silicon to form a silicide is from the group molybdenum, chromium,tantalum, titanium, and tungsten.

The fiber coating is at least sufficiently thick to be continuous andfree of significant porosity. Coating thickness can range from about 0.1micron to about 5 microns, and typically it is about 1.5 microns forfibers about 100 to 200 microns in diameter. The particular thickness ofthe coating is determinable empirically, i.e. it should be sufficient sothat the second layer prevents reaction, or prevents significantreaction, between the fibers and the infiltrating silicon under theparticular processing conditions used, and the first layer minimizesreaction between the matrix and the fiber during use of the composite,for example in high temperature service with oxidizing conditions.During the infiltration process, the second layer may or may not reactwith or dissolve in the molten silicon depending on time andtemperature, i.e., the second layer will survive better at lowertemperatures and for shorter times of infiltration. Generally, siliconinfiltration time increases with the size of the preform. Therefore,larger-sized preforms may require thicker fiber coatings.

The second layer should be free of significant porosity and preferablyis pore-free. Preferably, the second layer is uniform and smooth.Generally, the thickness of the second layer ranges from about 500Angstroms to about 3 microns, and typically it is about 0.5 microns. Theparticular thickness of the second layer is determinable empirically anddepends largely on the rate of consumption of the second layer, if any,and the particular composite desired.

The second layer is a solid which covers the first layer and adheressufficiently to form a coating thereon. Throughout the infiltrationprocess the second layer remains a solid. The second layer can promotecontact or wetting to improve the infiltration by capillarity, provide adesirable debonding with the matrix, protect the metal oxide coatingfrom the molten silicon infiltrant, or reduce reaction between thematrix and the fiber during high temperature service.

The metal reacting with silicon to form a silicide, and the silicide,must have melting points higher than the melting point of silicon andpreferably higher than about 1450° C. Generally, the metal and silicidethereof are solid in the infiltration process. Representative of suchmetals is chromium, molybdenum, tantalum, titanium and tungsten.

Known techniques can be used to deposit the second layer, for example,the second layer can be deposited by chemical vapor deposition using lowpressure techniques, or physical vapor deposition techniques such assputtering. For example, the metal carbide or metal silicide coating canbe directly deposited from the vapor thereof.

Alternatively, the metal carbide coating can be formed in situ byinitially depositing carbon followed by depositing metal thereon underconditions which form the metal carbide. If desired, metal silicidecoating can be produced by initially depositing the metal followed bydeposition of silicon under conditions which form the metal silicide.

A number of techniques can be used to determine if the fiber coatingsurvived. For example, if the composite exhibits fiber pull-out onfracture, then the fiber coating has survived. Also, scanning electronmicroscopy of a cross-section of the composite can detect the fibercoating on the fiber.

A carbonaceous material is admixed with or deposited on the coatedfibers to produce a mixture that is formed into a preform. Thecarbonaceous material is at least comprised of material which is wettedby molten silicon. The carbonaceous material as well as any reactionproduct thereof produced in the infiltration process should not flow toany significant extent and preferably is solid in the infiltrationprocess. The particular composition of the carbonaceous material isdeterminable empirically and depends on the particular compositedesired, i.e. the particular properties desired in the composite.However, the carbonaceous material contains sufficient elemental carbonto react with the infiltrating silicon to produce the compositecontaining silicon carbide formed in situ in an amount of at least about5% by volume of the composite. Generally, elemental carbon ranges fromabout 5% by volume, or from 10% or 20% by volume, to about 100% byvolume, of the carbonaceous material.

As used herein, the term elemental carbon includes particles, flakes,whiskers, or fibers of amorphous, single crystal, or polycrystallinecarbon, graphite, carbonized plant fibers, lamp black, finely dividedcoal, charcoal, and carbonized polymer fibers or felt such as rayon,polyacrylonitrile, and polyacetylene.

The carbonaceous material can be in the form of a carbon vaporinfiltration formed coating, a powder, a fibrous material or acombination thereof. When the carbonaceous material is in the form of apowder, it preferably has an average particle size of less than about 50microns, preferably, the carbon particles have a particle size of about1 to 15 microns, and most preferably, have a particle size of about 1 to5 microns. The carbon powder serves as a source of carbon to react withthe infiltrant and form silicon carbide, and as a binder to maintain theshape and integrity of the preform. However, an excessive volumefraction of carbon powder particles can cause swelling and cracking ofthe infiltrated composite. The carbon powder particles can have adensity of about 1.2 to 2.2 grams per milliliter. Preferably, the carbonpowder particles are a low density amorphous carbon having a density ofabout 1.2 to 1.6 grams per milliliter. A suitable carbon powder is aDylon aqueous graphite powder suspension, Dylon Industries, Inc., Ohio.Other sources for carbon powder are Johnson Matthey, Mass., and GreatLakes Carbon, N.Y. The amount and type of carbonaceous material dependslargely on the particular composite desired and is determinableempirically.

Preferably, the preform contains some fibrous carbon in the form ofchopped fibers of whiskers. The whiskers promote infiltration by wickingmolten silicon into the preform and are a source of carbon for reactingwith the infiltrant to form silicon carbide. Long whisker lengths aredesirable to achieve good wicking, while short whisker lengths result inbetter packing and less porosity to fill in the preform. The whiskersalso provide strength to the preform. Chopped fibers or whiskers can bedescribed by the aspect ratio of the fiber, fiber length to diameter.Preferably, the whiskers have an average aspect ratio that promoteswicking, and packs with the other components in the preform to providethe desired porosity in the preform. Preferably, the whiskers have anaverage diameter that allows complete reaction with the molten silicon.For example, a suitable whisker has an aspect ratio of about 5 to 50,and a fiber diameter of about 0.5 to 25 microns. Most preferably, theaspect ratio is about 5 to 15 and the whisker diameter is about 3 to 10microns. The whiskers can be graphitic, or preferably, amorphous. Thewhiskers have a density of about 1.2 to 2.2 grams per milliliter,preferably, about 1.2 to 1.6 grams per milliliter. Low density furnaceinsulation type WDF carbon felt, available from Union Carbide, can becrushed and abraded against a wire mesh screen, for example about 40mesh, to form suitable whiskers. Low density carbon fiber can be formedby carbonizing naturally occurring cellulose fibers, including cotton,chitosan, and bamboo, and chopped or crushed to form the whiskers.

The carbonaceous material also may include a metal which reacts withelemental silicon to form a silicide. Representative of such a metal ismolybdenum, chromium, tantalum, titanium, tungsten and zirconium. Themetal may comprise up to about 25% by volume of the carbonaceousmaterial, and preferably about 5% by volume.

The carbonaceous material may also include a ceramic material, in anamount up to about 50 percent by volume of the carbonaceous material.The ceramic material may or may not react with silicon, and is a ceramicsuch as a ceramic carbide, a ceramic nitride or a ceramic silicide. Theceramic carbide is selected form the group consisting of boron carbide,molybdenum carbide, niobium carbide, silicon carbide and titaniumcarbide. The ceramic nitride is selected from the group consisting ofaluminum nitride, niobium nitride, silicon nitride, titanium nitride andzirconium nitride. The ceramic silicide is selected from the groupconsisting of chromium silicide, molybdenum silicide, tantalum silicide,titanium silicide, tungsten silicide and zirconium silicide.

The carbonaceous material is admixed with the coated fibers in a mannerthat minimizes damage to the fiber coating of metal oxide, and ifpresent, minimizes damage to the second layer of coating on the fiber.Mixing can be carried out in a known and conventional manner. In oneembodiment, a slurry of the carbonaceous material can be cast in a moldcontaining the coated fibers to form a mixture. The slurry can be anorganic slurry containing known bonding means, such as for example epoxyresin, to aid in forming the preform.

The mixture can be formed or shaped into a preform or compact by anumber of known techniques. For example, it can be extruded, injectionmolded, die-pressed, isostatically pressed or slip cast to produce thepreform of desired size and shape. Preferably, the preform is of thesize and shape desired of the composite. Generally, there is nosignificant difference in dimension between the preform and theresulting composite. Any lubricants, binders, or similar materials usedin shaping the mixture are of the type which decompose on heating attemperatures below the infiltration temperature, preferably below 500°C., without leaving a residue that degrades the infiltration ormechanical properties of the resulting composite. It should beunderstood a suitable binder may leave a porous carbon deposit that doesnot degrade the infiltration or mechanical properties of the resultingcomposite.

The preform has an open porosity ranging from about 25% by volume toabout 90% by volume of the preform, and the particular amount of suchopen porosity depends largely on the particular composite desired.Frequently, the preform has an open porosity ranging from about 30% byvolume to about 80% by volume, or from about 40% by volume to about 60%by volume, of the preform. By open porosity of the preform, it is meantherein pores, voids or channels which are open to the surface of thepreform thereby making the interior surfaces accessible to the ambientatmosphere or the infiltrant. Preferably, the preform has no closedporosity. By closed porosity it is meant herein closed pores or voids,i.e. pores not open to the surface of the preform and therefore not incontact with the ambient atmosphere. Void or pore content, i.e. bothopen and closed porosity, can be determined by standard physical andmetallographic techniques.

Preferably, the pores in the preform are small, ranging from about 0.1micron to about 50 microns, and are distributed uniformly through thepreform thereby enabling the production of a composite wherein thematrix phase is uniformly distributed through the composite.

The preform is contacted with silicon-associated infiltrating meanswhereby silicon is infiltrated into the preform to form an infiltrationformed silicon carbide matrix. The infiltrating means allow silicon tobe infiltrated into the preform. For example, a structure or assembly isformed comprised of the preform in contact with means that are incontact with silicon and which permit infiltration of molten siliconinto the preform. In one infiltration technique, the preform is placedon a woven cloth of elemental carbon, a piece of silicon is also placedon the cloth, and the resulting structure is heated to infiltrationtemperature. At infiltration temperature, the molten silicon migratesalong the cloth and wicks into the preform. After infiltration, thewicking carbon cloth may be removed from the composite by diamondgrinding.

In another technique, the silicon infiltration procedure can be carriedout as set forth in U.S. Pat. No. 4,626,516, incorporated herein byreference, which discloses an assembly that includes a mold withinfiltration holes and a reservoir holding elemental silicon. Thepreform is placed within the mold and carbon wicks are provided in theinfiltrating holes. The wicks are in contact with the preform and alsowith the silicon and at infiltration temperature the molten siliconmigrates along the wicks into the preform.

U.S. Pat. No. 4,737,328 incorporated herein by reference, disclosesanother infiltration technique which comprises contacting the preformwith a powder mixture composed of silicon and hexagonal boron nitride,heating the resulting structure to a temperature at which the silicon isfluid and infiltrating the fluid silicon into the preform. Afterinfiltration, the resulting porous hexagonal boron nitride powder isbrushed off the composite.

Preforms having a simple square or rectangular shape can be infiltratedby placing silicon directly on the preform, and heating to a temperatureat which the silicon is fluid. The molten silicon wicks into andinfiltrates the preform.

The preform and infiltration structure or assembly are heated to theinfiltration temperature in an inert atmosphere or partial vacuum.Suitable inert atmospheres include argon, or reducing atmospheres suchas hydrogen or carbon monoxide. Atmospheres that react with moltensilicon, such as oxygen or nitrogen, are avoided. The remainingatmosphere of the partial vacuum should be inert, such as argon, orreducing such as carbon monoxide. Preferably, the partial vacuum isprovided before heating is initiated. The partial vacuum is at leastsufficient to avoid the entrapment of pockets of gas, and minimizesporosity in the infiltration formed composite. Generally, such a partialvacuum ranges from about 0.01 torr to about 2 torr, and usually fromabout 0.01 torr to about 1 torr to remove gas evolving in the preformbeing infiltrated.

Preferably, the furnace used is a carbon furnace, i.e., a furnaceconstructed essentially from elemental carbon. Such a furnace reactswith oxygen in the furnace atmosphere to produce CO or CO₂ and therebyprovides a nonoxidizing atmosphere so that reaction between the residualgas, preform, and infiltrant is minimized. Infiltration cannot becarried out in air because the liquid silicon would oxidize to form adense silica coating before any significant infusion by siliconoccurred. When a carbon furnace is not used, it is preferable to have amaterial that reacts with oxygen, such as elemental carbon, present inthe furnace chamber in order to provide a nonoxidizing atmosphere.Alternatively, other nonoxidizing atmospheres inert to the infiltrationprocess can be used at partial vacuums of about 10⁻² torr to 2 torr.

Infiltration is performed at a temperature where silicon is molten, butbelow the temperature where the silicon infiltrant begins to damage thefibers or metal oxide coating on the fibers. Molten silicon has a lowviscosity. The melting point of the silicon can vary depending largelyon the particular impurities which may be present. The infiltrationtemperature ranges from about 1400° C. to about 1550° C., and preferablyfrom about 1425° C. to about 1450° C. The rate of penetration of thesilicon into the preform depends on the wetting of the preform by thesilicon melt, and the fluidity of the melt. As the infiltrationtemperature increases, the ability of the molten silicon to wet thepreform improves.

Sufficient silicon is infiltrated into the preform to react with thepreform and produce the infiltration formed silicon carbide matrix.Specifically, the molten silicon is mobile and highly reactive withelemental carbon, i.e. it has an affinity for elemental carbon, wettingit and reacting with it to form silicon carbide. The molten silicon alsohas an affinity for the metals with which it reacts to form silicides.In addition, sufficient silicon is infiltrated into the preform to fillpores or voids which may remain in the composite.

The preform can be infiltrated with substantially pure silicon, ormolten silicon comprised of the metal in the coating on the fiber up toan amount that does not degrade the infiltration or mechanicalproperties of the resulting composite. As used herein, the term "moltensilicon" means essentially elemental silicon and up to about 10 atomicpercent, preferably up to 5 atomic percent, and most preferably up to 1atomic percent of the metal in the coating on the fiber, i.e., titanium,chromium, rhodium, iridium, zirconium, hafnium, aluminum, niobium,tantalum, boron, molybdenum, or tungsten. Infiltration with moltensilicon comprised of the metal in the coating on the fiber reduces orminimizes reaction between the infiltrating molten silicon and the fibercoating.

The period of time required for infiltration by the silicon isdeterminable empirically and depends largely on the size of the preformand extent of infiltration required. Generally, it is completed in lessthan about 20 minutes, and often in less than about 10 minutes.

The resulting infiltrated body is cooled in an atmosphere and at a ratewhich minimizes oxidation, cracking, or other defect formation withinthe body. Preferably it is furnace cooled in the inert atmosphere orpartial vacuum to about room temperature, and the resulting composite isrecovered.

The infiltration formed composite has a porosity of less than about 20%by volume, preferably less than about 10% or 5% by volume, and morepreferably less than about 1% by volume, of the composite. Mostpreferably, the composite is void or pore-free or has no significant orno detectable porosity. Preferably, any voids or pores in the compositeare small, preferably less than about 50 microns or less than about 10microns, and are substantially uniformly distributed in the composite.Specifically, any voids or pores are uniformly distributed throughoutthe composite so that they have minimal effect on the mechanicalproperties of the composite.

The composite of this invention is comprised of coated fibers and amolten silicon infiltration formed silicon carbide matrix. The matrix isdistributed through the coated fibers so that the matrix is spacefilling and interconnecting. Preferably, the coated fibers are totallyenveloped by the matrix. The fibers comprise at least about 5% byvolume, or at least about 10% by volume of the composite. The matrixcontains a silicon carbide phase formed in situ in an amount of about 5%to 90% by volume, or about 10% to 80% by volume, or about 30% to 60% byvolume, or about 45% to 55% by volume, of the composite. The matrix maycontain an elemental silicon phase in an amount of about 1 to 30% byvolume of the composite. The silicon carbide phase is distributedthroughout the composite, and preferably, it is distributed uniformly.

In one embodiment, the elemental silicon phase in the matrix is free oftitanium, zirconium, hafnium, aluminum, niobium, tantalum, boron,molybdenum, or tungsten. In another embodiment, the molten silicon hasan element from the group titanium, zirconium, chromium, rhodium,iridium, hafnium, aluminum, niobium, tantalum, boron, molybdenum, andtungsten dissolved therein ranging from a detectable amount up to about10 atomic percent, preferably up to 5 atomic percent, and mostpreferably up to 1 atomic percent of the elemental silicon phase. Moresensitive techniques such as microprobe analysis or Auger electronspectroscopy may be required to detect or determine the amount oftitanium, zirconium, hafnium, aluminum, niobium, tantalum, boron,molybdenum, or tungsten dissolved in the silicon phase.

The infiltration formed matrix may contain a phase of a metal silicideof molybdenum, chromium, tantalum, titanium, tungsten, or zirconium upto about 30 percent by volume of the composite. The metal silicide isdistributed throughout the composite, and preferably, it is distributeduniformly.

The infiltration formed matrix may contain a phase of a metal whichforms a silicide but which had not reacted with the infiltratingsilicon. In such instance, it would be encapsulated by a metal silicidephase. Such metal can range from about 0.5% by volume to about 5% byvolume, of the composite. The metal is distributed throughout thecomposite, and preferably, it is distributed uniformly.

The infiltration formed matrix may contain a phase of a ceramic materialfrom the group of ceramic carbide, ceramic nitride, or ceramic silicidediscussed above. The ceramic material may comprise up to about 50% byvolume, or from about 1% by volume to about 30% by volume, of thecomposite. The ceramic material is distributed throughout the composite,and preferably, it is distributed uniformly.

The infiltration formed silicon carbide matrix may contain a phase ofelemental carbon which has a significant amount of graphitic structure,i.e. a less reactive type of carbon, which had not completely reactedwith the infiltrating silicon. In such instance, this type of carbonwould be totally encapsulated by a phase of silicon carbide formed insitu. Such graphitic structure-containing elemental carbon generally canrange from about 0.5% by volume to about 10% by volume, or from about 1%by volume to about 5% by volume, of the composite. The graphiticstructure-containing elemental carbon is distributed throughout thecomposite, and preferably, it is distributed uniformly.

The composite is at least bonded by silicon carbide formed in situ. Itmay also be bonded by a metal silicide which formed in situ. It may alsobe bonded by elemental silicon or a bond formed in situ between siliconand a ceramic material.

The coated fibers in the composite are coated with the first layer ofmetal oxide that is detectable by scanning electron microscopy, and canrange in thickness from a detectable amount to about 5 microns,frequently from about 0.5 microns to about 1.5 microns. The second layercoating of rhenium, iridium, metal carbide, metal silicide, metalnitride, and metal diboride may survive the infiltration and remain aspart of the coating on the fibers. When the second layer was one of themetals that reacts with silicon to form a silicide, the metal reactswith the molten silicon infiltrant and becomes part of the matrix.

The particular amount of fiber coating in the composite depends largelyon the amount of coated fibers present, the thickness of the coating,and the diameter of the fiber. Therefore, the volume fraction of fibercoating is the balance of the volume fraction of all other components ofthe composite. However, in one embodiment, the fiber coating in thecomposite ranges from less than about 1% by volume to about 30% byvolume, or from about 1% by volume to about 10% by volume, of the totalvolume of fibers. In another embodiment, the fiber coating ranges fromless than 1% by volume to about 20% by volume, or from about 1% byvolume to about 5% by volume, of the composite.

The fiber component ranges form about 5% by volume to less than about75% by volume, or from about 10% by volume to about 70% by volume, orfrom about 15% by volume to less than about 65% by volume, or from about25% by volume to about 50% by volume, of the composite. The coatedfibers are distributed throughout the composite, and most often, it isdistributed uniformly throughout the composite. However, in some casesit is desirable to have higher packing fractions of the coated fibers inregions of the composite where higher local strength or stiffness may bedesired. For example, in a structure having a long thin part, such as avalve stem, it is advantageous to strengthen the stem by increasing thevolume fraction of the coated fibers in the stem region of thestructure.

The coated fibers in the composite impart significant toughness to thecomposite. Specifically, the coated fibers minimize brittle fracture ofthe composite at room temperature, i.e. 25° C. By brittle fracture of acomposite it is meant herein that the entire composite cracks apart atthe plane of fracture. In contrast to a brittle fracture, the presentcomposite exhibits fiber pull-out on fracture at room temperaturebecause the fiber coating provides a desirable debonding of the fiberfrom the matrix. Specifically, as the present composite cracks open,generally at least about 10% by volume, frequently at least about 50% byvolume and preferably all of the metal oxide-coated fibers do not breakat the plane of fracture, but instead pull out of the matrix. In thisway, a stress transmitted through the composite by a crack in the matrixis distributed along the length of fibers in the path of the crack.Distribution of stress along the length of the fibers greatly diminishesthe stress at the crack tip and reduces propagation of the crack throughthe matrix.

One particular advantage of this invention is that the composite can beproduced directly in a wide range of sizes and shapes which heretoforemay not have been possible to manufacture or which may have requiredmachining operations. For example, the composite can be as short asabout an inch or less, or as long as desired. It can be of simple,complex, or hollow geometry. For example, it can be produced in the formof a tube or a hollow cylinder, a ring, a sphere, or a bar having asharp point at one end. Since the composite can be produced in apredetermined configuration of predetermined dimensions, it requireslittle or no machining.

The composite has a wide range of applications depending largely on itsparticular composition. It can be used, for example, as a wear resistantpart, bearing or tool insert, acoustical part, or high-temperaturestructural component.

The invention is further illustrated by the following examples where,unless otherwise stated, the following materials and equipment wereused. The preform binder was comprised of "Epon 828" which contained acuring agent. "Epon 828" is a resin formed from the reaction ofepichlorohydrin and Bisphenol A, which is a liquid at room temperatureand which has an epoxide equivalent of 185-192. Epon 828 decomposescompletely below 1300° C. The curing agent was diethylenetriamine, aliquid commonly called DTA which cures Epon 828 thereby solidifying it.The carbon resistance furnace used to form the composite was containedin a vacuum belljar system.

EXAMPLE 1

A continuous yttrium oxide coating of about 1 micron was deposited bysputtering on 140 micron diameter silicon carbide fibers, trade nameSCS-0, obtained from Textron, Mass. The yttria coated fibers were coatedwith a solution of iridium resinate, dried for 1 hour, and heated in airto 400° C. for 5 minutes to convert the resinate to metallic iridium. Aslurry was prepared by hand-stirring, by weight, 11 parts carbon powder,17 parts of a solution comprised of epoxy resin and xylene in a one toone ratio, and 0.88 parts DTA catalyst. The coated fibers were laid in avacuum-casting mold and the slurry was poured over and around thefibers. Liquid in the mold was removed by applying a vacuum of about 100torr to one end of the mold. The remaining preform with the coatedsilicon carbide fibers embedded therein, was dried under an infraredlamp for 24 hours to cure the epoxy.

The dried preform was transferred into a vacuum furnace and set on apiece of carbon cloth. An amount of silicon, about three time the weightof the carbon in the preform and the carbon cloth, was placed on thecarbon cloth so that the silicon was not in direct contact with thepreform. The preform was heated at about 2° C. per minute to 500° C. topyrolize the epoxy, and heated at about 10° C. per minute to 1425° C.The temperature was maintained at 1425° C. for 15 minutes allowing thesilicon to melt and infiltrate the preform. The infiltrated preform wasfurnace cooled. Metallographic inspection of a polished-cross-section ofthe composite showed there was no visible degradation of the fiber, witha distinct boundary present between the matrix and the fiber coating.

EXAMPLE 2

A single crystal alumina rod about 1.2 millimeters in diameter wasincorporated into a preform, and infiltrated with silicon as describedabove in Example 1. Metallographic inspection of a polishedcross-section of the composite showed circumferential cracks in thematrix around the interface between the fiber and the matrix. Crackswere also visible within the alumina rod.

EXAMPLE 3

A continuous iridium coating of about 2 microns was deposited bysputtering on a single crystal alumina fiber, about 150 microns indiameter. The coated alumina fibers were incorporated into a preform,and infiltrated with silicon as described above in Example 1, to form afiber reinforced composite. Metallographic inspection of a polishedcross-section of the composite showed a small number of circumferentialcracks in the matrix at the interface between the matrix and coatedfiber.

In Example 2, it is believed the cracking observed in the matrix andalumina rod was due to strong bonding between the alumina rod and thematrix, and the mismatch in thermal expansion between the alumina rodand matrix. The alumina rod has a higher thermal expansion as comparedto the matrix, so that upon cooling from the infiltration temperature,the alumina rod pulls away from the matrix. Because the alumina rodstrongly bonds with the matrix, a tensile stress was developed as thealumina rod pulled away from the matrix, and caused the cracking.Example 3 shows that a coating of a silicide forming metal on the oxidealumina fiber, results in greatly reduced cracking in the matrix becausethere is minimal bonding between the oxide and the matrix. Therefore, asilicon carbide fiber having a coating of a first layer of alumina and asecond layer of a silicide forming metal provides a desirable debondingbetween the silicon carbide fiber and the infiltration formed matrix.

We claim:
 1. A fiber reinforced composite body comprising from about 5to 90 volume percent of a continuous silicon carbide matrix phase,formed in situ by molten silicon infiltration and reaction betweenmolten silicon and a carbonaceous material, and at least about 10 volumepercent of reinforcing fibers consisting essentially of silicon carbidefibers, said fibers having a continuous first coating layer on thesilicon carbide fiber surface of a metal oxide selected from the groupconsisting of aluminum oxide, yttrium oxide, titanium oxide, zirconiumoxide, hafnium oxide, beryllium oxide, silicon oxide, lanthanum oxide,and scandium oxide, whereby the silicon carbide fiber coated with themetal oxide first layer is protected against the effects of moltensilicon during infiltration and after infiltration provides a compositehaving a relatively weak bond at a fiber-matrix interface, saidcomposite having the porosity of less than 20 volume percent, and saidmatrix containing a silicon carbide and an elemental silicon phase, saidsilicon phase in an amount of about 1 to 30 percent by volume of thecomposite.
 2. A fiber reinforced body according to claim 1 wherein themetal oxide coating is substantially pore-free and has a thickness ofabout 0.6 to 5 microns, the metal oxide coated fiber has a diameter ofabout 0.3 to 150 microns and an aspect ratio of at least about
 10. 3. Acomposite comprised of reinforcement fibers from the group consisting ofelemental carbon, silicon carbide, and mixtures thereof; where saidfibers have a continuous first coating layer of metal oxide from thegroup consisting of aluminum oxide, yttrium oxide, titanium oxide,zirconium oxide, hafnium oxide, beryllium oxide, silicon oxide,lanthanum oxide, scandium oxide, and mixtures thereof; and a siliconcarbide ceramic matrix formed by molten silicon infiltration, where saidmatrix contains at least a silicon carbide phase and an elementalsilicon phase.